Optical projection systems such as televisions and computer monitors use cathode ray tubes (CRTs) as displays. A liquid crystal on silicon, or LCOS, light modulator, is an alternative display component that has some advantages over CRTs. In particular, LCOS light modulators are flat, thus occupying less space, and use less power than CRTs.
LCOS displays consist of layered components. A surface layer of glass or transparent plastic substrate is disposed over a middle layer of liquid crystal material, which is further supported by an underlying layer of silicon substrate, known as a back plane. The glass or transparent plastic layers are manufactured with transparent electrodes on the surface adjacent to the liquid crystal material. A patterned metal layer is further disposed upon the back plane, comprising hundreds or thousands of reflecting mirrors, known as micro-mirrors.
The LCOS display thus comprises a first glass or plastic layer, an electrode layer, a liquid crystal layer, a second electrode layer, a patterned metal film layer, and a second glass or plastic layer. A pixel of the LCOS display includes a single micro-mirror, along with its associated layers. The LCOS display is thus an array of pixels, including hundreds or thousands of individual pixels.
LCOS displays use polarized lenses to control the transmission of light, known as light modulation, to individual pixels within the pixel array. LCOS displays are thus sometimes referred to as light modulators. The electrodes embedded in the glass or plastic layers allow an electric field to be applied across the liquid crystal material. Molecules in the liquid crystal medium exhibit polarization alignment when subjected to the electric field. The liquid crystal thus acts as a dynamic polarizing medium.
One of the glass or plastic layers of the LCOS display is coated to respond as a static polarizing filter. Together the two polarizing filters modulate the light received by individual pixels of the LCOS display. When the pixel is turned “off,” the light attempting to transmit through the pixel is blocked by the polarizing filters. When the pixel is turned “on,” the light travels through the polarizing filters unimpeded, reflecting off the micro-mirror.
Due to minute manufacturing inconsistencies or defects, the materials that make up the LCOS light modulator do not have a precise thickness at the microscopic level. These differences in material thickness cause differences in the thickness of the cell gap, or space between the two glass or plastic layers, in which liquid crystal material occupies the cell gap.
To more precisely control cell gap spacing, many manufacturers spray minute glass or plastic beads over the patterned metal film. The glass or plastic beads, or spacers, are meant to ensure that the cell gap is uniform all along the pixel array, thus improving image quality. However, the glass or plastic beads, which share space with the liquid crystal material in the cell gap, may interfere with light traveling through the liquid crystal. Further, when the pixel is turned “on,” and the image is magnified, the interference caused by the beads is also enlarged, often appearing as a defect in the display. While the defect may not be apparent on an optical projection system from a distance, it is more likely visible at close proximity to the discriminating eye.
Because of these issues, some LCOS manufacturers are making spacerless displays, with unsatisfying results. Generally, the quality of spacerless displays is poor. Other manufacturers control the cell gap spacing by etching spacers into the overlying glass or plastic plate. This etching technique, while more precisely controlling spacing between the pixels of the display, may continue to adversely affect image quality, as the spacers block the transmission of light through the LCOS media.
Thus, there is a need for an LCOS display in which spacers precisely control the cell gap, but which do not interfere with light transmission through each pixel of the display.